In the prior art adapter of this type, two legs of the adapter are linked through a swivel and are pressed against each other under spring tension, in a manner similar to that of clothespins. The contacts engage the conductors of the embraced contact rail under spring tension. An essential disadvantage of this known adapter lies in the fact that it is not secured against unintentional displacement, since retention is achieved solely through spring force. Furthermore, the fact that the electrical contacts also represent the mechanical holding elements has proven to be a disadvantage.